Polydiorganosiloxane fluids having a low silanol content exhibit benefits such as improved viscosity stability at elevated temperatures as a consequence of improved thermal stability. Reaction with a silylating agent such as hexamethyldisilazane provides a method of removing silanol chainstopping groups. In contrast the direct production of low silanol content polydiorganosiloxane fluids from silanol containing feeds is not as well understood.
The primary chemical route to tri-organosilyl endblocked polydiorganosiloxanes involves the equilibration or polymerization of cyclopolydiorganosiloxanes, silanol chainstopped linear polydiorganosiloxanes, or mixtures thereof with an appropriate chainstopping agent such as hexaorganodisiloxane or a short chain length polydiorganosiloxane fluid in the presence of a catalyst. Suitable catalysts for this polymerization or equilibration include both acids and bases. The product of this process is a mixture of the desired product contaminated by a lesser amount of the starting material(s).
The earliest catalysts used for this process were soluble acids or bases. Because the catalytic agent was soluble, deactivation or separation of the catalyst from the reaction products presented difficulties in downstream purification. Further, very strong acid catalysts such as sulfuric acid created problems with undesirable side reactions, such as cleavage of the organic substituents from the silicone in the polyorganosiloxane.
An early solution to the problems presented by soluble catalysts, was the use of solid catalysts. This is accomplished by resorting to the practices of heterogeneous catalysis and bonding the catalyst to a support, or alternatively using a solid material having catalytic properties, e.g. ion exchange resins. As a practical matter, ion exchange resins have been unsatisfactory from the standpoint of requiring long residence times and in addition are fairly expensive by comparison to alternative catalytic materials such as sulfuric acid and the like.
Both acid treated carbon black and acid treated clays have been used in fixed bed processes. These materials suffer from the drawback that practical conversions require fairly high temperatures. Depending on the product desired and the equilibrium relationships involved this is a greater or lesser drawback. These processes utilized temperatures ranging anywhere from 85.degree. to 200.degree. C. and were frequently operated at reduced pressures ranging from 5 to 200 mm Hg, particularly in the case of acid treated clays which were used in powdered, as opposed to granular, forms, because of pressure drop problems across the catalyst bed. At the lower temperatures, side reactions were minimized, but low pressures, i.e. partial vacuums, have been necessary to produce low silanol equilibrates from reactants containing high levels of silanol.
In order to reduce the problems created by increasing temperature to increase the reaction rate, the use of two beds in series was implemented (Siciliano et al. U.S. Pat. No. 3,853,934). When two fixed beds are used in series, it became possible to use less active materials such as acid treated clays as the catalysts for the equilibration polymerization reaction. The reduced activity of the acid treated hydroaluminum silicate clay catalyst was partially compensated for by operating both catalyst beds at temperatures ranging from 150.degree. to 200.degree. C. Early developers of these processes generally were not particularly concerned with the silanol content of the resulting product nor whether the product was primarily linear or contaminated with a significant amount of branched product.
Later developments teaching a complete reversal of some of the preferred process parameters, e.g. a granular catalyst particles as opposed to finely divided catalyst particles, resulted in process improvements: 1) reducing the temperature range of operation to 100.degree. to 150.degree. C. and 2) an essentially water-free product (Elms, U.S. Pat. No. 4,831,174). Even with these improvements, the process requires two fixed beds operated in series when utilizing acid treated clays.